diff --git a/.github/workflows/claude.yml b/.github/workflows/claude.yml
index 8aa4ba71a..f68ae5275 100644
--- a/.github/workflows/claude.yml
+++ b/.github/workflows/claude.yml
@@ -35,13 +35,18 @@ jobs:
       id-token: write
       actions: read
     steps:
+      # Actions are pinned to full commit SHAs rather than mutable major tags:
+      # this job holds ANTHROPIC_API_KEY and grants the agent Bash, so a
+      # force-moved tag would be an unreviewed code change inside a
+      # secret-holding job. The trailing `# vX.Y.Z` comment is the form
+      # Dependabot reads, so pinning costs us no upgrade automation. (#1882)
       - name: Get PR details
         if: |
           (github.event_name == 'issue_comment' && github.event.issue.pull_request) ||
           github.event_name == 'pull_request_review_comment' ||
           github.event_name == 'pull_request_review'
         id: pr
-        uses: actions/github-script@v8
+        uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0
         with:
           script: |
             let prNumber;
@@ -57,26 +62,58 @@ jobs:
               pull_number: prNumber
             });
 
+            // A fork PR's head lives in a different repository — or in none at
+            // all, if the fork was deleted after the PR was opened (`head.repo`
+            // is then null). Neither is ours to check out, so both count as a
+            // fork here and the steps below decline rather than guess.
+            const headRepo = pr.data.head.repo?.full_name ?? null;
+            const isFork = headRepo !== `${context.repo.owner}/${context.repo.repo}`;
+
             core.setOutput('sha', pr.data.head.sha);
-            core.setOutput('repo', pr.data.head.repo.full_name);
+            core.setOutput('is_fork', String(isFork));
+
+      # A fork PR's head is untrusted code, and checking it out would put it in
+      # reach of a tool-enabled agent run holding ANTHROPIC_API_KEY. Reviewing
+      # the base tree instead would only trade that for a confident review of
+      # the wrong tree, so decline visibly and leave the reason in the run.
+      - name: Decline fork PR
+        if: steps.pr.outcome == 'success' && steps.pr.outputs.is_fork == 'true'
+        run: |
+          {
+            echo "### Claude Code declined this pull request"
+            echo
+            echo "The head branch lives in a fork, so its code is not checked out and Claude is not run."
+            echo "Push the branch to this repository and re-trigger if a review is needed."
+          } >> "$GITHUB_STEP_SUMMARY"
 
+      # No `repository:` — a same-repo head is all that reaches this step, and
+      # checkout defaults to `github.repository`, which no PR can influence.
       - name: Checkout PR branch
-        if: steps.pr.outcome == 'success'
-        uses: actions/checkout@v6
+        if: steps.pr.outcome == 'success' && steps.pr.outputs.is_fork == 'false'
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           ref: ${{ steps.pr.outputs.sha }}
-          repository: ${{ steps.pr.outputs.repo }}
           fetch-depth: 0
 
+      # `skipped` means the trigger was an issue or a non-PR comment, so there
+      # is no head to check out and the base tree is the right one. The lookup
+      # having *failed* is deliberately not included: this condition carries no
+      # status-check function, so GitHub applies an implicit `success()` and
+      # skips the step after a failed prior step anyway. Spelling out `skipped`
+      # keeps that from having to be re-derived by the next reader.
       - name: Checkout repository
-        if: steps.pr.outcome != 'success'
-        uses: actions/checkout@v6
+        if: steps.pr.outcome == 'skipped'
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           fetch-depth: 0
 
       - name: Run Claude Code
+        # Runs only against a tree this workflow actually checked out: the base
+        # repo (non-PR trigger) or a same-repo PR head. A fork PR reaches here
+        # with `is_fork == 'true'` and both disjuncts false, so it is skipped.
+        if: steps.pr.outcome == 'skipped' || steps.pr.outputs.is_fork == 'false'
         id: claude
-        uses: anthropics/claude-code-action@v1
+        uses: anthropics/claude-code-action@5ef2e550a465a721f4f45e4a7d3c340c873e1dcc # v1.0.190
         with:
           anthropic_api_key: ${{ secrets.ANTHROPIC_API_KEY }}